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Khairi M. Salem. B.Pharm. PhDCP.AU

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Complexometric titrationsComplexometric titrations Complexometric titrations are those titrations

involving the formation of a class of substances known as “Complex”

Uses of Complex-formation reactions : One of the first uses of these reagents was for

titrating cations. Many complexes are colored or absorb ultraviolet

radiation; the formation of these complexes is often the basis for spectrophotometric determinations.

Some complexes are sparingly soluble and can be used in gravimetric analysis.

The most useful complexometric agents are organic compounds that contain several electron donor groups that form multiple covalent bonds with metal ions

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FORMATION of COMPLEXESFORMATION of COMPLEXES

Most metal ions react with electron-pair donors to form coordination compounds or complexes. The donor species, or ligand is an ion or a molecule that forms a covalent bond with a cation or a neutral metal atom by donating a pair of electrons that are then shared by the two.The number of covalent bonds that a cation tends to form with electron donors is its Coordination numberCoordination number. Typical values for coordination numbers are two, four, and six.

M M (METAL ION) (METAL ION) + L + L (LIGAND) (LIGAND) ======= ML ======= ML ( COMPLEX)( COMPLEX)

The complex formed my be electrically positive, negative or neutral , thus for example copper(II) can form a positive complex with ammonia Cu(NH3)4

2+ , a neutral with glycine Cu(NH2CH2COO)2 and negative with chloride ion CuCl42-

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TYPES OF LIGAND:TYPES OF LIGAND:1. A unidentate ligand: that has a single donor group, such as

ammonia, is called unidentate (single-toothed), 2. A multidentate ligand: when the ligand is attached to the

metal ion in two or more points as bidenate. Tridentate, tetradentate, pentadentate, and hexadentate ligands

Producing ComplexesProducing ComplexesComplexation reactions involve a metal ion M reacting with a ligand L to form a complex ML.

M + L MLComplexation reactions occur in a stepwise fashion, and the reaction above is often followed by additional reactions:

ML + L ML2

ML2 + L ML3

MLn-1 + L MLn

Unidentate ligands invariably add in a series of steps. With multidentate ligands, the maximum coordination number of the cation may be satisfied with only one or a few added ligands.

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Chelation:Chelation:Compounds containing two or more donor groups

combine with the metal ion to form ring structure called ”ChelatesChelates”. The ligand is called ”Chelating Chelating agentagent” and the products are called ”Metal Chelates”. ”Metal Chelates”.

TThe chelate forming ligand must have at least two acidic groups, or two coordinating groups or one acidic group and one coordinating groups; examples of acidic groups such as:

-OH Phenolic, - SO2H Sulphinic, -SO3H Sulphonic and -OH Phenolic, - SO2H Sulphinic, -SO3H Sulphonic and COOH Carboxylic groups.COOH Carboxylic groups.

Examples of coordinating groups containing N, O, or S N, O, or S such as:

C=O Carbonyl, OH hydroxyl, S sulphide, SO2 sulphone, C=O Carbonyl, OH hydroxyl, S sulphide, SO2 sulphone, NH2 amino, NO nitroso, NO2 nitro and =N- cyclic NH2 amino, NO nitroso, NO2 nitro and =N- cyclic nitrogennitrogen

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Complex Equilibrium Constants The equilibrium constants for complex formation reactions are generally written as formation constants.

M + L ML = [ML]/[M][L] = K1

M + 2L ML2

M + 3L ML3

M + nL MLn

The overall formation constants are products of the stepwise formation constants for the individual steps leading to the product.

2

21 2

ML

M LK K

3

31 2 3

ML

M LK K K

n

nn

ML

M LK K K

1 2. . . .

2

3

n

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Ethylenediaminetetraacetic Acid (EDTA)Ethylenediaminetetraacetic Acid (EDTA)Ethylenediaminetetraacetic acid [also called (ethylenedinitrilo)tetraacetic acid], which is commonly shortened to EDTA, is the most widely used complexometric titrant. Fully protonated EDTA has the structure

The EDTA molecule has six potential sites for bonding a metal ion: the four carboxyl groups and the two amino groups, each of the latter with an unshared pair of electrons. Thus, EDTA is a hexadentate ligand.

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EDTA Is a Tetrabasic AcidEDTA Is a Tetrabasic Acid

The dissociation constants for the acidic groups in EDTA are K1 = 1.02 X 10-2, K2 = 2.14 X 10-3, K3 = 6.92 X 10-7, and K4 = 5.50 X 10-11 . It is of interest that the first two constants are of the same order of magnitude, which suggests that the two protons involved dissociate from opposite ends of the long molecule. As a consequence of their physical separation, the negative charge created by the first dissociation does not greatly affect the removal of the second proton. The various EDTA species are often abbreviated H4Y, H3Y-, H2Y2-, HY3-, and Y4-.

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Complexation Titration CurvesComplexation Titration Curves

The progress of a complexometric titration is generally illustrated by a titration curve, which is usually a plot of pM = -log[M] as a function of the volume of titrant added. Most often in complexometric titrations the ligand is the titrant and the metal ion the analyte, although occasionally the reverse is true. Many precipitation titrations use the metal ion as the titrant. Most simple inorganic ligands are unidentate, which can lead to low complex stability and indistinct titration end points.

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…continued…

As titrants, multidentate ligands, particularly those having four or six donor groups, have two advantages over their unidentate counterparts. First, they generally react more completely with cations and thus provide sharper end points. Second, they ordinarily react with metal ions in a single-step process, whereas complex formation with unidentate ligands usually involves two or more intermediate species.

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Reagents for EDTA TitrationsReagents for EDTA TitrationsThe free acid H4Y and the dihydrate of the sodium salt,

Na2H2Y.2H2O, are commercially available in reagent quality.

The Nature of EDTA Complexes with Metal IonsSolutions of EDTA are valuable as titrants because the reagent combines with metal ions in a 1:1 ratio regardless of the charge on the cation.

Ag+ + Y4- AgY3-

Al3+ + Y4- AlY-

EDTA is a remarkable reagent not only because it forms chelates with all cation but also because most of these chelates are sufficiently stable for titrations. This great stability undoubtedly results from the several complexing sites within the molecule that give rise to a cagelike structure in which the cation is effectively surrounded and isolated from solvent molecules. The ability of EDTA to complex metals is responsible for its widespread use as a preservative in foods and in biological samples

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Indicators for EDTA TitrationsIndicators are organic dyes that form colored chelates with metal ions in a pM range that is characteristic of the particular cation and dye. The complexes are often intensely colored and are discernible to the eye at concentrations in the range of 10-6 to 10-7 M.Eriochrome Black T is a typical metal-ion indicator used in the titration of several common cations.

H2O + H2In- HIn2- + H3O+ K1 = 5 X 10-7

red blue

H2O + HIn2- In3- + H3O+ K2 = 2.8 X 10-12

blue orange

The acids and their conjugate bases have different colors.

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…continued…

The metal complexes of Eriochrome Black T are generally red, as is H2In-. Thus, for metal-ion detection, it is necessary to adjust the pH to 7 or above so that the blue form of the species, HIn2-, predominates in the absence of a metal ion. Until the equivalence point in a titration, the indicator complexes the excess metal ion so that the solution is red. With the first slight excess of EDTA, the solution turns blue as a consequence of the reaction

MIn- + HY3- HIn2- + MY2-

red blue

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Titration Methods Employing EDTA

Direct Titration: Many of the metals in the periodic table can be determined by titration with standard EDTA solution. Some methods are based on indicators that respond to the analyte itself, whereas others are based on an added metal ion.

Methods Based on Indicators for an Added metal Ion: In case where a good, direct indicator for the analyte is unavailable, a small amount of a metal ion for which a good indicator is available can be added. The metal ion must form a complex that is less stable than the analyte complex.

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…continued…Potentionmetric Methods: Potential measurements can be used for end-point detection in the EDTA titration of those metal ion for which specific ion electrodes are available.

Spectrophotometric Methods: Measurement of UV/visible absorption can also be used to determine the end points of titrations. In these cases, an instrument responds to the color change in the titration rather than relying on a visual determination of the end point.

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…continued…Back-Titration Methods: Back-titrations are useful for the determination of cations that form stable EDTA complexes and for which a satisfactory indicator is not available; the determination of thallium is an extreme example. The method is also useful for cations such as Cr(III) and Co(III) that react only slowly with EDTA. A measured excess of standard EDTA solution is added to the analyte solution. After the reaction is judged complete, the excess EDTA is back-titrated with a standard magnesium or zinc ion solution to an Eriochrome Black T or Calmagite end point.

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…continued…

Displacement methods: In displacement titrations, an unmeasured excess of a solution containing the magnesium or zinc complex of EDTA is introduced into the analyte solution. If the analyte forms a more stable complex than that of magnesium or zinc, the following displacement reaction occurs:

MgY2- + M2+ MY2- + Mg2+ where M2+ represents the analyte cation. The liberated Mg2+ or, in some cases Zn2+ is then titrated with a standard EDTA solution. Displacement titrations are used when no indicator for an analyte is available.

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